In third-generation W-CDMA-based systems (Wideband Code Division Multiple Access), such as UMTS, subscribers are separated by using subscriber-specific spread codes. In this case, each symbol in a subscriber-specific data sequence is multiplied by the spread code. The elements of the resultant sequence are called chips. Each subscriber-specific physical transmission channel is in this case allocated a dedicated spread code, also called channelization code. The subscriber-specific physical channels are each spread using a channelization code. For a UMTS system, the generation of the transmission signal in the downlink, i.e. from the base station to the mobile stations, is described in the UMTS-standard document 3GPP TS 25.213 V5.3.0.
The channelization codes are “OVSF (Orthogonal Variable Spreading Factor) spread codes”. These are described in the UMTS-standard document 3GPP TS 25.213 V5.3.0 under Section 4.3. The various OVSF spread codes are orthogonal with respect to one another and can have various code lengths and various spread factors. The OVSF spread codes are selected from an OVSF code tree. This has a plurality of levels, whose associated OVSF spread codes are characterized by the same spread factor. Each OVSF spread code with a spread factor n is followed in the OVSF code tree by two mutually orthogonal OVSF spread codes with the spread factor 2n, but these codes are not longer orthogonal with respect to the OVSF spread code with the spread factor n. To ensure the orthogonality of the spread-coded signals, only particular OVSF spread codes are therefore permitted to be selected from the OVSF code tree: as soon as an OVSF spread code from the OVSF code tree with a particular spread factor is already being used, all spread codes with a higher spread factor which follow this OVSF spread code in the OVSF code tree are no longer permitted to be used.
After the individual physical channels have been spread, the resultant channels have a chip rate of 3.84 MHz. Next, the spread signals are coded with a scrambling code, and the chip rate remains the same. Generally, this involves the same scrambling code being used in a base station for all channels. Following subsequent power scaling, the individual channels are overlaid through addition to form a total signal. Alternatively, instead of the separate coding of the individual channels with the scrambling code, it is also possible to code the overlaid total signal with the scrambling code. In addition, the total signal is overlaid with subscriber-independent synchronization channels. The resultant complex signal is then subjected to pulse shaping and is then up-converted to the carrier-frequency band. After that, the signal is fed into a linear-operation power amplifier and is then radiated via the antenna.
The power of the total signal coded with the scrambling code has a wide dynamic range. Typically, the dynamic range is approximately 10 dB.
Such a wide dynamic range has a negative effect particularly on the power amplifier, in which linear operation needs to be ensured over the entire dynamic range. This therefore needs to be designed to be of corresponding size.
Patent Specification U.S. Pat. No. 5,991,262 discloses a technical doctrine whose aim is to reduce the dynamic range of the input signal for the power amplifier in a CDMA-based system. In this context, the power of a signal compiled from a plurality of differently spread-coded signals is limited through overlaying with a correction signal. Spreading is not performed using OVSF spread codes in this case, but rather using “Walsh codes” with a constant spread factor. The correction signal is formed by first producing a provisional correction signal from the compiled signal. Walsh code-domain transformation of the compiled signal is used to ascertain the quantity of orthogonal spread codes which are already being used. With knowledge of the spread codes which are already being used for the compiled signal, it is possible to remove from the provisional correction signal those signal components which are based on spread codes which are already being used. There thus remain only the signal components which are based on spread codes which are not yet being used. The resultant correction signal is then overlaid with the compiled signal, so that the signal power of the resultant signal is limited.
Laid-Open Specification WO 02/101954 A1, which forms the closest prior art, describes a similarly operating solution for limiting the power in a W-CDMA system. The spread codes underlying both the compiled signal and the correction signal are the aforementioned OVSF spread codes. Instead of through Walsh code-domain transformation, the quantity of OVSF spread codes which are already being used is ascertained by means of OVSF code-domain transformation. The OVSF code-domain transformation takes place for a particular spread factor SFmin.
A drawback of the solution described is that the correction signal is formed by using OVSF spread codes with the spread factor SFmin, the spread factor SFmin corresponding to the smallest spread factor for the spread codes used in the compiled signal. If, by way of example, the compiled signal contains a data channel at a high data rate which requires a spread factor of 8, then SFmin=8 and there thus remain a maximum of 7 OVSF spread codes for producing the correction signal. If the compiled signal additionally also contains further data channels at a low data rate, i.e. with a high spread factor, some or even all of the 7 remaining OVSF spread codes are eliminated as free OVSF spread codes during OVSF code-domain transformation with the spread factor SFmin, of the compiled signal. If all OVSF spread codes with the spread factor SFmin are eliminated as free OVSF spread codes, it is not possible to generate a correction signal which is orthogonal with respect to the compiled signal.
In addition, the prior art takes account neither of the influence of the coding with the scrambling code nor of the influence of a transmission-end pulse-shaping filter or of the two synchronization channels. Furthermore, the technical doctrine known from the prior art can be applied only when just one scrambling code is used in a base station; the use of different scrambling codes is thus not taken into account.